Friday, March 21, 2014

Tilting & Shifting with Magic

My Tilt + Shift lens experiments continue and each time I
use it my confidence grows. Also, as it was a bit of a risk to buy this
expensive lens, ie I didn’t know I would like/use it, I am keen to find out
earlier, rather than later, that I am indeed going to keep it.

Some of you have said to me: why bother with such a lens?

That is: why buy such an expensive lens, after all if you wish
to correct for perspective, ie shift, you can do this via pano-bracketingand Photoshop stitching. If you wish to get
greater control over focus in a landscape, say, then you can use
focus-bracketing and merge in Photoshop. Others have said that you can even
emulate some of the the tilt effect using Photoshop filters.

Well, yes and no; and here is my simple thinking:

By having a 24mm Tilt+Shift lens, I have, first
of all got myself a very ‘sharp’ 24mm prime lens. In addition, because it is a
shift lens, the diameter of the glass is much larger than in a ‘normal’ 24mm
prime, thus the corner-to-corner sharpness, at the unshifted point, is higher
than a normal 24mm Prime. At first look the price difference between the Canon
24mm F/1.4L prime and the 24mm TS-E F3.5L is not ‘much’, however, it is not
really fair to compare these two lens, eg autofocus vs no autofocus, F/1.4 vs
F/3.5, plus no-tilt+shift vs tilt+shift. Having considered the differences, and
wishing to have a wide high Image Quality (IQ) prime; for me, the TS-E was a
better choice than the 24mm F/1.4L; and I couldn’t justify both!

In normal lenses the single plane of sharp focus
is fixed parallel to the sensor plane. Either side of this the focus decreases
parallel to the plane of sharp focus. As we all know, if you wish to ensure a
scene is ‘tack sharp’ throughout,
ie from near to far, then, according to your viewing criteria, you will choose an
appropriate exposure setting and follow one of three basic focusing strategies.
You could simply focus at the hyperfocal point; you could focus at infinity (or
the furthest point you wish to see in focus) and stop down the aperture to
capture the detail in the near field; or you could adopt a landscape, ie not
macro, focus-bracketing strategy and merge the resultant images in Photoshop
(or some other focus stacking software).

All the above focusing strategies have one
weakness in common. They all only ensure the scene is ‘perfectly’ focused at
one (or a few in the case of focus-bracketing) plane(s) in the image, parallel
to the sensor, and which may not be coincident with the ‘visual hotspot(s)’ in
the image. When we use (full field) hyperfocal, infinity or bracketing focusing
we choose various image compromises. For instance, we select a, so-called,
Circle of Confusion, which is related to our sensor and depth of focus (sic)
requirements, eg (print vs screen) viewing needs. For example, for a 35mm DSLR,
if you follow ‘web-standard-advice’, you may select a CoC of .030mm, or 0.033. But
in many cases you may not even be able to select the CofC, as, say, the hyperfocal
calculator you are using predefines a fixed CofC for you, eg irrespective of
your final printing needs. Many of these hyperfocal calculators ignore
diffraction (but see here www.georgedouvos.com/douvos/Intro_to_TrueDoF-Pro.html
), thus you may think you are getting greater depth of field as you stop down
from, say, F/8 to F/16 or F/22; but when you examine your print, you are
disappointed to see the image is soft. For prints, you really need to know the
print size (and viewing distance) to be most informed on CoC selection. Thus
for a given image capture and CoC selection criterion, a screen view of an
image may look OK; and even a 6x4 print may look OK; but what if your mother
wants the image blown up to, say, 24x16 or larger? Without some pre-thinking
about the final presentation, keep your fingers crossed!

This is where the tilt function of a Tilt+Shift
lens scores: that is being able to move the plane of sharp focus literally
anywhere you wish, eg bringing spatially disparate elements in a scene into common
focus sharpness: as long as they remain within the acceptable focus wedge.
Thus, say, being able to bring a near, middle and foreground landscape element
into the tack sharp focus, ie on the same plane of (perfect) focus, in a single
capture; and, if you wish, being able to include out of focus areas to help
bring the viewers’ attention to the ‘important’ elements in the image. Of
course nothing in life is free and the ‘downside’ to the tilt lens is the need
to manage the resultant focus wedge, as opposed to a more simple (parallel to
the sensor) focus slice. But, I would contest, that, once you understand the
‘science’ behind the tilt shift positioning the plane and wedge of focus is
fairly simple. The tilt lens also allows you to keep the lens at the sweet spot
of the aperture, say, F8 to minimize defocus blur and diffraction, and,
compared to a non-tilt lens, bring greater control to the in and out of focus
areas of the image. For the landscape photographer it is the ultimate tool.

The
shift function of the lens allows you to, relatively simply, capture perfect
(‘zero’ distortion) panos, either 1-D or 2-D, ie horizontal/vertical or
‘square’. This in-camera ability pushes the 24mm to a much wider lens
equivalence, thus my TS-E 24mm prime, with a little post processes is also
about a 14.5 mm lens, which, on a full frame camera is impressive!

However,
the star feature is that the shift allows perspective correction; which, for
architectural work, is essential. Yes, you can achieve such corrections in Photoshop,
however, even before you start post-processing for such things as colour, luminance,
dodging and burning etc, you need to destructively re-map the image to correct
for perspective (unless of course your image works leaving it skewed or leaning!).
In a shift lens no pixels are harmed in the creation of a pano or when creating
a perspective corrected image!

The tilt function also allows one to
create ‘special/artistic’ focus effects, for instance the so-called miniature effect. But I personally haven’t played with this yet,
so I will not speak of it any more here.

Finally,
some have said it is all too complicated to get the best exposure with a
tilt+shift lens, as, as soon as you tilt or shift, your in-camera metering is
compromised. Well, for the Canon users, I have an answer: Magic Lantern
Auto-ETTR mode. In other words, just before you press the shutter, simply
invoke A-ETTR and ML chooses the best (ETTR) exposure!

As usual, the above represents my humble opinions. Each
person will need to decide for themselves what photography tools they need. For
me, although it is early days, I see myself making good use of my new (fully
featured) 24mm F3.5L prime lens.